snapshot of the cellulosic energy grasses

Whenever I mention what I work on, nobody has ever heard of Miscanthus. Of course I consider my friends, family, & acquaintances to be pretty well-informed smart people, so this seemed a bit surprising. This lack of awareness about the incredible potential of Miscanthus & other crops as biofuel crops has been a big reason why I started this blog.

In order to simplify and clarify, I decided to make a brief list & descriptor (in no particular order) of the major energy crops, focusing on the perennial grasses. There are also non-grass cellulosic energy crops (such as poplar, willow, Jatropha and Agave--see previous blog post), which I will not go into here. Disclaimer: this is hardly an exhaustive description of all of these grasses, just what I choose to include...

1) Miscanthus (known sometimes as Elephant grass or Amur silver grass, this includes mainly the sterile hybrid Miscanthus x giganteus, but also relatives such as M. sinensis, M. sacchariflorus, M. floridulus). Tropical to temperate. Native to Asia & Africa.


Advantages: No fertilizer (nitrogen) input required, low degree of invasiveness (hybrid M. x giganteus is sterile), cold-tolerant, highly productive (see photo above!!), a carbon neutral source of fuel when life-cycle is considered, lots of natural variation in M. sinensis & M. sacchariflorus, can grow on marginal lands
Disadvantages: high initial planting costs (must plant rhizomes rather than seeds, due to sterility), M.x giganteus plants typically grown are all cuttings from a single genetic clone (greater genetic variation is typically favored), fairly high water needs during growing season--limits use in arid western U.S.

2) Switchgrass (commonly known as panic grass, Panicum virgatum) Native to North America.
Advantages: cold-tolerant, fairly drought-tolerant, relatively low fertilizer inputs needed, a carbon neutral source of fuel when life-cycle is considered, highly diverse, can grow on marginal lands
Disadvantages: roughly half as productive as Miscanthus in most climates, relatively high water needs during warm growing season (spring/summer) which limits use in much of the western U.S.

3) Maize (corn). Native to North America. Annual rather than perennial.
Advantages: well-established as an ethanol crop when using seeds, annual crops have some benefits, can use the stover (leaf tissue) as a byproduct of seed production for cellulosic ethanol production
Disadvantages: very high fertilizer and water input needs--leading to high carbon costs, not as prolific as dedicated cellulosic perennial grasses, growth for ethanol production competes with growth for food

4) Sugarcane (many species of Sachharum) Tropical to warm temperate climates, Native to South Asia. Brazil is the largest grower of sugarcane, where they generate ethanol as a by-product of sugar production. Brazil is self-sufficient in terms of fuel production due to this investment in sugarcane-based ethanol.
Advantages: high sugar content--sugar is directly fermented into ethanol,
Disadvantages: very water intensive, grows in tropical (warm) climates only thereby limiting its growth in the primarily temperate U.S., not typically grown for cellulosic biofuel production

5) Energy cane (sugarcane hybrids produced to make low sugar varieties).
Advantages: hybrids (crop behind person in above photo) often have increased vigour and are highly prolific, the energy cane varieties are bred to be more cold-tolerant than sugarcane which increases the growing range in the U.S., can potentially convert to ethanol in the same way as other cellulosic biofuel crops, can be more productive than sugarcane (producing more ethanol per unit of land)
Disadvantages: unless a "no sugar" variety is developed, separate conversions to ethanol are necessary (e.g., different process to convert sugar to ethanol vs. converting cellulose to ethanol), relatively high water needs, initial planting costs are high/intensive (established from cuttings rather than seeds), not cold-tolerant

6) Sweet sorghum (many varieties of Sorghum with high sugar content). Native to tropical and sub-tropical regions on all continents (except Antarctica)
Advantages: fairly high yield, relatively drought tolerant, direct conversion to ethanol from sugar (not typically cellulose)
Disadvantages: grown primarily for sugar conversion directly to ethanol rather than as a cellulosic form of ethanol production...but could be used for both (like energy cane), annual rather than perennial, high fertilizer needs, susceptible to pests, not cold tolerant


7) Native prairie (many species, primarily Switchgrass [Panicum virgatum], Indiangrass [Sorghastrum nutans], Eastern Gamagrass (Tripsacum dactyloides), Big Bluestem (Andropogon gerardii), Little Bluestem (Schizachyrium scoparium), and others)
Advantages: high biodiversity, excellent habitat for wildlife, renewable, requires no fertilizer or irrigation
Disadvantages: generates only a fraction of the productivity compared with dedicated energy crops

life-cycle analysis of greenhouse gas emissions

We already know that most of our energy comes from fossil fuels AND fossil fuels spew tons of greenhouse gases (GHG) that contribute to global warming, but are biofuels actually much better?? The answer is...it depends.

Energy analysts and economists are now looking at the entire life-cycle of producing fuels (which includes extraction of fuel, or planting to processing, transport, and the burning of fuels as emissions). Gasoline is usually used as the reference, and ideally, other forms of fuels emit LESS greenhouse gases than gasoline when the entire life cycle is taken into account.

In this 2009 life-cycle analysis of different types of fuels (http://www.pnas.org/content/106/6/2077.full), Hill et al. found that corn based ethanol produced as much or even more greenhouse gas emissions (including CO2, N2O, and CH4), than gasoline, a surprising result at first. But the GHG emissions differed in production of corn ethanol depending on the source of heat at the biorefinery (whether it was fueled by natural gas, coal, or corn stover--the left-over "leaf" part of a corn plant after the fruits are harvested).

As a bright point, cellulosic ethanol has significantly reduced levels of GHG emissions relative to either corn ethanol or gasoline, when the life-cycle of the fuel production is accounted for. Of note, however, is that the data used to estimate emissions for cellulosic biofuels is relatively limited as this form of ethanol production has not been done to scale.

Costs of GHG (A) and particulate matter: PM2.5 (B) emissions. Per liter and per gallon estimates are shown alongside total costs arising from production of an additional billion gallons of ethanol or an energy-equivalent volume of gasoline. (C) Combined costs of GHG and PM2.5. From PNAS article by Hill et al. 2009. http://www.pnas.org/content/106/6/2077.full

Why is corn-based ethanol not an improvement over gasoline with respect to fossil fuel emissions? The answer is that corn is a fairly intensive crop, it needs a lot of nitrogen fertilizers , and has higher fossil fuel input, all of which contribute to increased GHG emissions. Cellulosic ethanol production is better, when the life-cycle is accounted for, because it requires little to no fertilizer and lignin combustion from the cellulosic crops provides excess heat and power at the biorefinery, which displaces fossil fuel and electricity consumption.

While cellulosic biofuels are a large improvement over conventional forms of corn-ethanol and gasoline in terms of GHG emissions, there is rarely such thing as a golden ticket. That is, there are trade-offs to GHG reductions in terms of other forms of air pollution. From studies (http://dancingflames.org/dancingflames/EnvSci/Articles/EnvScipdffiles/EthanolPublicHealth.pdf, http://www.afdc.energy.gov/afdc/pdfs/technical_paper_feb09.pdf) looking at E85 blend fuels (85% ethanol blended with 15% gasoline) compared to straight gasoline, emissions are reduced for several greenhouse gases, but emissions INCREASE for other pollutants (such as ethanol, formaldehyde, and acetaldehyde). Both formaldehyde and acetaldehyde are nasty carcinogens, as classified by the U.S. EPA. Unburned ethanol can also oxidize to acetaledehyde, so these air pollutants (while not classified as greenhouse gases) have potential human health risks.

Like most things, there is no simple single solution. And it would be good to see ways to reduce both GHG emissions AND other forms of air pollution if we want to move forward with renewable fuel sources. Sorry, but the grass is not always cleaner.

Carbon-free energy

Globally, 87% of our energy comes from fossil fuels. These are dense sources of energy (see previous post on why oil tastes so good) & oil transports and stores well...but what about greenhouse gas emissions? Reams of evidence have demonstrated that greenhouse gases, such as CO2 (carbon dioxide), in excess, contribute to the overall warming of our planet. And fossil fuels produce A LOT of CO2. Another problem with fossil fuels is that they are a finite resource. Some estimates say that only ~100 years of oil resources remain (Source: World Energy Assessment 2000 & 2004/UNDP ). http://www.undp.org/energy/activities/wea/drafts-frame.html

Renewable energy sources (such as solar, wind, biomass, and geothermal), on the other hand, have very low CO2 output per kilowatt-hour of energy produced (See figure at http://www.sciencemag.org/cgi/reprint/329/5993/786.pdf). But given the amount of energy we currently use, is there enough renewable energy to meet this demand? The single largest potential comes from solar energy, where the total amount of solar energy available on earth's surface is several orders of magnitude greater than what we currently use, as a planet. And there is a good chance we could capture enough of that to generate the amount of power the world now consumes. One of the current challenges with solar, however, is energy storage. Sunlight does not shine in one solar panel 24/7, and we need more inexpensive, more efficient ways to store this energy when it is not being generated. There are many great leads to building better batteries out there..but I am not an engineer and will pass on further comment for now.

Biomass can be thought of as another source of solar energy, as plants convert sunlight into energy that is stored in the plant as sugar (via photosynthesis). Even though plant growth is seasonal, with an abundance of plant material (feedstock) at the end of the summer growing season, the leaves can be harvested and stored until energy is needed. A disadvantage of biomass, however, is that is requires resources such as water and land.

While there is much promise in renewables replacing fossil fuels, most experts agree that the best future outcome will include a number of different renewable energy sources and technologies.

Bioenergy crops in the desert??

There is some concern that bioenergy crops are more trouble than they're worth. They may produce a lot of biomass, but doesn't that come at a cost of lots of added fertilizer and water? Are fertile, midwestern climates the only suitable locations for growing crops, for food or fuel? For some plants, that may be true. But different plants are well-adapted to arid & semi-arid landscapes.

One group of plants with a fairly long history of cultivation includes various Agave species. Agave plants are succulents with a special type of photosynthesis (a process plants use to convert sunlight into energy) called CAM (Crassulacean acid metabolism) that increases water-use efficiency relative to most other plants. Traditionally, Agave varieties have many different uses: they have been used for production of fibers like sisal, for honey/sugar substitutes (agave nectar), and the flowers and stalks are edible. The most well-known use for agave is the fermentation & distillation of the sap to make mezcal, one type of which is tequila.

In addition to these uses, the leaves are also good sources of biomass that can be used as feedstock for conversion into biofuels. Although Agave farms may not produce as much biomass (in terms of tons per hectare) as Miscanthus or Sugarcane (see table below), Agave plants can be productive with as little as 12% of the water needed by these large grasses.

Table: Estimated productivity, rainfall, and nitrogen requirements of current or potential bioenergy crops (see article by Somerville et al. 2010. Science 329:790-792 for references).

Crop	Average productivity (MT ha–1 year–1)	Ethanol yield (liter ha–1)	Seasonal water requirements (cm year–1)	Tolerance to drought	Nitrogen requirements (kg ha–1 year–1)
Corn		3800 (total)	50–80	low	90–120
Grain	7	2900
Stover	3	900
Sugarcane	80 (wet)	9950 (total)	150–250	moderate	0–100
Sugar	11	6900
Bagasse	10	3000
Miscanthus	15–40	4600–12,400	75–120	low	0–15
Poplar	5–11	1500–3400	70–105	moderate	0–50
Agave spp.	10–34	3000–10,500	30–80	high	0–12

There is a lot of interest in producing energy sustainably in this country. Water is a particularly valuable resource, particularly in the western U.S. Using excessive irrigation or groundwater to grow crops in arid regions is not a responsible use of this resource. However, many plants, such as Agave, are naturally adapted to climates that experience little and/or erratic rainfall. By planting and harvesting climatically-appropriate energy crops, even the more extreme regions of this diverse country could become good producers of bioenergy.

Re: funding innovation

I recently attended the Philomathia Foundation Symposium at Berkeley on "Pathways to a Sustainable Energy Future," which had a number of amazing speakers. I was particularly impressed with Arun Majumdar, the director of ARPA-E (Advanced Research Projects Agency-Energy, sponsored by the Dept. of Energy. ARPA-E's mission is explicitly bold: to fund potentially revolutionary technologies that are too risky for industry to fund. Another mission is to re-assert the United States' technological leadership.

I naively assumed that the U.S. is already at the top of their game technologically, but Majumdar pointed out that the majority of the leading "green" energy companies (solar companies, electric car manufacturers, advanced rechargeable battery manufacturers) are foreign. This may be because, as Manumdar says, "The U.S. spends more on potato chips than on Energy R&D." It seems we are addicted to what is bad for us (see previous post on oil addiction). I agree that we should rediscover some national pride in ingenuity and technology and re-assert our technological leadership. Promising research results can have a high impact commercially, ideally in the form of smart, socially and environmentally responsible capitalism (a source for another blog post: the academic-industry partnership).

Even though it is probably not enough, I am really excited by the government's commitment to fund these "big ideas." Although it's considered risky, I think we live too much in a society that is scared to take risks, scared to innovate, because of the chance of failure. Sure, most of these ventures won't pan out, but even if just a single innovative, pie-in-the-sky idea works, it could make today's technology obsolete.

Coping with the "blend wall"

Part of the problem with biofuels gaining any momentum in our current car-driven, fossil fuel based economy is that our engines are built to run on gasoline..which can be blended with 10% ethanol. If the U.S. uses ~140 billion gallons of gasoline a year, then the demand for ethanol is about 14 billion gallons annually. Apparently we already produce about 12 billion gallons of corn ethanol per year (with even more capacity in idled biorefineries)...so we have basically reached what is referred to as the "blend wall," with no market for cellulosic ethanol.

How to cope with this dilemma?

One option would be to replace all corn-based ethanol plants with cellulosic biofuel pruduction plants, which would be an expensive, short-term fix, essentially tearing down something fully functional. And the "blend wall" problem would remain, with a maximum market of 14 billion gallons.

Another option is to increase the required amount of ethanol in blended fuels to 12% or 15%, something the EPA is considering. This is the most likely scenario, as it's technically and politically safest. But truthfully, this option will not do a whole lot to encourage use of biofuels in the long-term. I think of this option as a baby step, not covering much ground, but better than nothing.

A third, bolder option would be to make the leap to flex-fuel car engines, such as those that run on E85 ethanol (a fuel blend of 85% ethanol & 15% gasoline). Most of the cars in Brazil are built to run using some level of blended fuel, up to 100% ethanol. Since Brazil generates all of their own ethanol from sugarcane refineries, they have attractively achieved energy security (a topic for a future blog). Estimates of the cost of producing all future car engines to be flex-fuel lays around $100/car. If the average cost per new car in 2009 sold in the U.S. (according to the National Automobile Dealers Association) is $28,400, this minor change would amount to less than 1% of the value of the car. Although flex fuel contains its own controversies, it seems like a bold alternative way forward that would encourage both greater energy security and economic stability for the cellulosic biofuels market.

Power density--why oil tastes so good..

As Americans, we are addicted to things that are bad for us. French fries, hot dogs, chips, sodas & cookies. All these foods contain loads of sugar & fat--essentially rich sources of energy for our bodies. The trouble is, we really don't need these rich sources of energy or "nutrients" in excess. One cheeseburger probably takes care of all of our daily caloric needs.

Same is true for oil. It is densely packed with energy. Nothing (other than coal and other petroleum products) comes even close to oil in terms of power density (the amount of energy produced per square meter of Earth's surface). Solar and wind facilities provide 1-2 orders of magnitude less power per square meter than oil, and biomass plants (based on corn ethanol) are even less dense...providing 1% (or less) of the energy per area compared to oil--see figure below from article in Science 329:780 (2010).

Source of data in figure: upper left and bottom--DOE; upper right--V. Smil, Energy Transitions, Praeger (2010)

Yikes! That's sobering news & tough to compete with. To soften the blow somewhat, it is worth pointing out that the full cost of energy should also include the cost of extraction, transport, storage, AND environmental risks (including CO2 output)...which is where renewables have the potential to come out ahead.

Can we learn anything from our unhealthy food addiction that can be applied to our unhealthy oil addiction? A good start is educating people on the scope of the problem, benefits & disadvantages to different forms of energy. But the truth is, most people do not want to change their lifestyles. An oft cited poll by The New York Times/CBS News (released 20 June 2010) found that although 90% of respondents agreed that "U.S. energy policy either needs fundamental changes or to be completely rebuilt," only 49% supported new taxes on gasoline to fund new and renewable energy. Seems to me that we should stop subsidizing oil and instead subsidize renewables, AND impose Greenhouse Gas taxes (on energy companies!!) that penalize for catastrophic environmental risks (such as drilling for oil) and emitting CO2 at any phase of the production. That might help level the playing field.